A typical gas turbine engine compressor includes several rows or stages of compressor stator vanes and corresponding rows or stages of compressor rotor blades therebetween. As ambient air flows through each succeeding compressor stage during operation, it is successively compressed for providing compressed air to a combustor located downstream therefrom wherein it is mixed with fuel and ignited for generating hot combustion gases which power the engine.
In order to improve the overall operation of the compressor, several compressor stator vanes are variable for allowing each vane to rotate around its longitudinal or radial axis to adjust the angular orientation of the vane relative to the airflow thereover. Variable stator vanes include an integral outer trunnion disposed in a complementary mounting boss in a stator casing for allowing angular adjustment of the vane relative to the airflow thereover. Extending outwardly from the vane trunnion in one exemplary embodiment is a coaxial threaded stem around which is seated and fixedly joined thereto by a retention nut a lever arm having a distal end pivotally joined to an actuation ring. All of the vane lever arms in a single row are joined to a common actuation ring for ensuring that all of the variable vanes are positioned relative to the airflow in the compressor stage at the same angular orientation. If the orientation of one or more vanes is different than the remainder of the vanes in the same stage, an aerodynamic distortion may be generated which corresponds to a one excitation per revolution of the downstream stage or rotor blades which may decrease the useful life thereof.
In order to accurately assemble the lever arms to the respective vane trunnions during initial assembly, or during field maintenance of the compressor, each trunnion typically includes a generally D-shaped seat at the juncture of the base of the stem and the top of the trunnion, with the proximal end of each lever arm having a corresponding D-shaped mounting hole. The mounting hole is typically slightly less in width than the width of the trunnion seat so that tightening of the nut on the stem forces the lever arm into a no tolerance or interference fit on the trunnion seat. In order to allow assembly of the mounting hole over the trunnion stem, the mounting hole typically includes opposing arcuate cutouts representing portions of a circle having a diameter slightly larger than the diameter of the trunnion stem. However, this allows the lever arm to be assembled downwardly over the stem without interference therewith, and correspondingly allows the lever arm to be misaligned with the trunnion seat unless suitable care is exercised for ensuring accurate alignment therebetween.
Although the corresponding D-shapes of the trunnion seat and lever arm mounting hole are provided for eliminating misalignment therebetween, the assembly process for obtaining the interference fit therebetween may nevertheless result in a misalignment when the interference fit is formed. Tightening the retention nut without confirming that the lever is oriented correctly with the vane stem flats can produce misalignments of any angle between the lever arm and its respective vane. This is possible since the force created by applying torque to the nut has the capability to produce plastic deformation in the parent metal of both the trunnion seat and the lever arm. Since this interface is covered by the retention nut, this occasional misalignment can go unnoticed during assembly.